The velocity of light is 3×108 m/s. Since this is known, the distance to an object can be measured by emitting light, receiving the light reflected from the object, and measuring the delay time. The TOF (Time-Of-Flight) method is a method for measuring the distance to an object by measuring the time of the flight of the light. Table 1 shows the relationship between the resolution of the measurement of the delay time and the resolution of the distance.
TABLE 1Relationship between delay time measurement resolutionand distance resolution based on the TOF methodDelay time measurement resolutionMeasurement distance resolution10μs1.5km1μs150m100ns15m10ns1.5m1ns15cm100ps1.5cm10ps1.5mm1ps0.15mm
According to Table 1, the distance measurement range with respect to the delay time measurement range can be read along with the distance measurement resolution, and if an equipment of which the delay time measurement range is 1 μs and the delay time measurement resolution is 1 ns could be available, then a 150 m range can be measured with a 15 cm resolution, and the equipment can be used as an on-vehicle distance sensor.
Three related prior arts have been reported.    (1) Inventor: Cyrus Bemji, Assignee: Canesta Inc. “CMOS-Compatible Three-dimensional Image Sensor”, U.S. Pat. No. 6,323,942 B1, Nov. 27, 2001    (2) R. Lange, P. Seitz, A. Biber, S. Lauxtermann, “Demodulation pixels in CCD and CMOS technologies for time-of-flight ranging”, Proceedings of SPIE, Vol. 3965, pp. 177 - 188, (2000)    (3) Ryohei Miyagawa, Takeo Kanade, “CCD-based range-finding sensor”, IEEE Trans. Electron Devices, Vol. 44, No. 10, pp. 1648-1652 (1997)
Method (1) concerns projecting a pulse light, shaping the waveform of the pulse by detecting the peak of the received signal pulse, and digitally measuring the delay time using a high-speed pulse. In this case, a sufficient brightness of light is required to generate a pulse from the received light signal, so application is limited.
Methods (2) and (3) are similar methods. Method (2) is implemented in a process where CCD and CMOS are integrated, where charges are alternately transferred to two nodes at high-speed synchronizing with a high frequency modulated light of 20 MHz, using the charge transfer of CCD, and measurement is performed by using the fact that the distribution ratio of the charges to two nodes depends on the delay time of the modulated light. Since a mixed process of CCD and CMOS is required, cost is high.
Method (3) concerns alternately transferring charges, generated by modulated light based on pulse modulation, to two nodes using the structure of CCD, and measurement is performed by using the fact that the distribution ratio thereof depends on the delay time of the modulated light. This also uses CCD, so special manufacturing steps are required.